Process for physical refinement of a liquid, especially a glass melt, and apparatus for performing said process

ABSTRACT

In many engineering production processes, for example glass or glass-ceramic manufacturing processes, liquids, such as glass melts, participate in the processes in which gases are dissolved, which in part form bubbles in the liquid. So that the quality of the final product is not disadvantageously influenced, the liquid, e.g. glass melt, should be refined to remove the bubbles. According to the method of the invention an overpressure acting on the liquid is provided which is such that the internal pressure in the bubbles immediately under the surface of the liquid in a refining chamber is at least as great as the sum of equilibrium pressures of the gases dissolved in the liquid and the sum of the vapor pressure of components evaporating from the liquid. A two stage apparatus is provided which performs a preferred embodiment of the refining method of the invention. This two stage apparatus includes a tower-like comparatively higher low pressure refining chamber ( 6 ) and a downstream comparatively lower overpressure refining chamber ( 1 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for physicalrefinement of a liquid, especially a glass melt, which has dissolvedgases and bubble-shaped gas inclusions, by varying the pressure on theliquid in a refining chamber.

[0003] The present invention also relates to an apparatus for physicalrefinement of a liquid, which has dissolved gases and bubble-shaped gasinclusions, with at least one processing container for treatment of theliquid and with at least one refining chamber in which there is a changeof the pressure exerted on the liquid relative to the pressure in theprocessing container.

[0004] 2. Prior Art

[0005] Liquids in which gases are dissolved, which, in part, formbubbles in the liquids, are participants in many engineering processes.Since these gases or gas bubbles can interfere with further processingor impair the properties and thus the quality of the manufacturedproduct disadvantageously, it is necessary to free the liquid from thesegas bubbles. This priority is designated as degassing or refinement.

[0006] In the following the refinement of a glass melt is used as anexample of the refinement of a liquid in general, but the inventionshould not be limited here to that example. The analogous problem arisesin many engineering applications.

[0007] As a result of the decomposition of the starting materials in theinitial batch considerable amounts of gases are generated during themelting of glass. As a crude estimate it is said that about 1 kg ofglass results from melting 1.2 kg of batch, i.e. during the meltingabout ⅕ of the batch weight is released in the form of gas. Other gasesphysically accompany the batch or starting mixture or are introduced bythe combustion heat source used in the glass melting process.

[0008] Most gases escape of course during the initial melting of theglass, however a considerably portion of the gases are entrapped in themelt. A portion of the trapped gases is dissolved in the glass melt,while the other portion remains as local gas inclusions, as so-calledbubbles, in the melt. The bubbles shrink or grow when the bubbleinternal pressure is higher or lower than the equilibrium pressure ofthe dissolved gas. The gas bubbles have different sizes.

[0009] Since these gas bubbles disadvantageously impair the quality of aglass or glass-ceramic body made from the glass melt, the glass meltmust be refined of the gas, i.e. which means that the gas must beremoved from the glass melt.

[0010] The term “refining of the glass” means a melt process stepsubsequently connected with the melting in the so-called refiningchamber, which

[0011] largely results in a removal of a definite size class of gasbubbles and

[0012] guarantees a desired adjustment of the gas content of the glassmelt and at the same time

[0013] is integrated in a complex sequence of melting process steps.

[0014] The refining of the glass is then of highest significance for thequality of the product resulting from the melting process.

[0015] A variety of methods have been used in the prior art for therefining.

[0016] The gas bubbles have the tendency to rise in the melt and toescape from containment into the surroundings because of their staticbuoyancy. This process however requires considerable time withoutexternal influence, which would be costly for the production processbecause of the long idle time. It is thus known to provide highertemperatures in a refining zone in order to increase the viscosity ofthe melt and thus the rising speed and bubble diameter of the gasbubbles. This additional temperature rise requires a considerable amountof energy, which adds comparatively large additional costs to theproduction process.

[0017] The methods of chemical refining of glass with oxides by means oftemperature-dependent oxidation steps are well tested and largelyoptimized. These methods especially involve refining agents such asNaCl, Sb(V)-oxide, As(V)-oxide and Sn(IV)-oxide.

[0018] During chemical refining especially the rising speed of smallerbubbles is increased because the refining gas O₂ that is generated fromthe refining agent is pumped into them.

[0019] Chemical refining methods comprise a sequence of elementary stepsinterwoven with each other in time and space. The finely dispersedbubbles in the crude melt are expanded to such a great extent by therefining gas O₂ that a drastic shortening of the rising time occurs.Simultaneously the refined bubbles extract gas dissolved in the glass.In subsequent cooling steps as complete as possible a resorption of theunavoidable remaining bubbles occurs. Among others, satisfactory color,water-content and concentration limits of O₂ and SO₂ are major goals fora successful control of the gas content of the glass. A once-achievedsatisfactory bubble quality may not be impaired again during cooling orshaping processes.

[0020] Chemical refining has several disadvantages in principle limitingit:

[0021] the method does not function for every glass system, especiallyin NaCl refining, or only at higher temperatures;

[0022] the refining process requires much time, since gas diffusionoccurs slowly in the melt. Because of that the refining chambers musthave a comparatively large extent which increases the production costs;

[0023] the refining agents change the chemistry of the glass and thusits properties; furthermore they are toxic (arsenic, antimony).

[0024] Because of these disadvantages the so-called physical refiningprocesses were introduced which do not disadvantageously change theglass chemistry. The physical refining of the glass melt is based on“forcing” the bubbles with physical methods to climb to the surface ofthe melt. The bubbles then burst at the surface and release their gascontent or dissolve in the melt.

[0025] The so-called low pressure refining is a physical refining methodin wide spread use, which is described in numerous literaturereferences, for example in U.S. Pat. No. 4,738,938 and European PatentDocument EP 0 231 518 B1.

[0026] In low pressure refining the bubbles present in the melt growwhen the equilibrium pressure on the gas dissolved in the melt dropsbelow their internal pressure (hydrostatic pressure in the melt plus thesurface tension pressure of the bubble). Because of that the bubblesbecome larger, growing more rapidly at the surface of the melt, burstingthere or are “skimmed off”. The growth speed is determined primarily bymelt-bubble gas transport.

[0027] The prior art methods of low pressure refining has the followingdisadvantages in principle limiting them:

[0028] a) The growth speed of the bubbles is primarily determined by gasdiffusion and by the equilibrium pressure of the gas dissolved in themelt; low pressure refining operates only economically then, i.e.sufficiently rapidly, when additional chemical refining agents are used.Thus low pressure refining usually does not permit the completeabandonment of chemical refining agents, but allows only a gradualreduction of the chemical refining agents. Thus U.S. Pat. Nos.4,886,539, 4,919,697 and 4,919,700 describe a method for vigorousbubbling of glass melts at low pressure for removal of gas ingredients.

[0029] b) Furthermore low pressure refining is based on the rise ofbubbles to the surface of the melt. Thus the viscosity of the melt mustbe sufficiently low or its temperature must be sufficiently high. Glassmelts being refined at refining conditions usually have a viscosity ofdefinitely less than 10 Pas, which means that the associatedtemperatures of the glass melts must certainly be over 1400° C., inspecial glasses definitely over 1500° C.; low pressure refining requiresa comparatively large-surface and long refining chamber and acomparatively long refining time, which disadvantageously effectsproduction costs.

[0030] c) Small bubbles grow only very slowly during low pressurerefining and mainly do not climb to the surface of the melt. All thebubbles are hardly removed from the melt by low pressure refining,furthermore only the bubble spectrum in the melt (gross dispersion ofbubbles) changes. The engineering goal is then to beneficially transformthe bubble spectrum. A complete debubbling or bubble removal from themelt is thus hardly possible.

[0031] d) If the hydrostatic pressure falls below the equilibriumpressure of the dissolved gas, a very vigorous spontaneous bubbleformation, i.e. a foam is formed, which is to be removed in anadditional process step. Thus U.S. Pat. No. 4,794,860, which correspondsto European Patent Document 0 253 188, describes a process in which thefoam or froth arising in a glass melt under low pressure is collapsed byaddition of foam-breaking substances, such as water, alkali metalcompounds, or solutions of NaOH or carbonates. European Patent DocumentEP 0 257 238 B1 describes a method in which the foam is collapsed byadditional heating of a region in the melt container above the meltedmaterial. U.S. Pat. No. 4,849,004 describes a method in which a glassmelt at low pressure is exposed to additional periodic pressureoscillations (<1 s duration for a pulse, 10 . . . 60 s duration for thetotal pulse train) in order to collapse the foam. All these measuresincrease the cost of the manufacturing process and disadvantageouslyeffect the glass chemistry in the above-named references.

[0032] e) The process parameters for the low pressure refining dependsensitively on the equilibrium pressure of the gas dissolved in themelt. The low pressure refining with stable process parameters requiresthe use of expensive raw material with uniform quality.

[0033] f) In low pressure refining a strong evaporation of components ofthe melt occurs. Because of that under certain circumstances the pumpsare strongly loaded or the composition of the melt changes. A temporaryout flow can even occur at high vapor pressure, which is veryunfortunate.

SUMMARY OF THE INVENTION

[0034] It is an object of the present invention to provide a process ofthe above-described kind and an associated apparatus so that theconcerned liquid can be refined in a simple manner with almost completeor even total avoidance of chemical refining agents, so that therefining is also effective in liquids with comparatively high viscosity,such as glass melts at comparatively lower temperatures than have beenpossible in prior art methods and that the refining time iscomparatively shorter than in prior art refining methods.

[0035] It is an additional object of the invention to provide a methodand apparatus for physically refining a glass melt to eliminatedissolved gasses and gas bubbles without or with reduced usage ofchemical refining agents.

[0036] According to the invention the liquid is acted on with anoverpressure that is adjusted or set so that the internal pressure inthe individual bubbles immediately under the surface of the liquid levelin the refining chamber or containment chamber is at least as great asthe sum of the equilibrium pressures of the gases dissolved in theliquid and the sum of the vapor pressures of the components evaporatingfrom the liquid.

[0037] Because of the features of the invention a pressure is producedin the bubbles, which is above the equilibrium pressure of the gasesdissolved in the liquid. Because of that the gases found in the bubblesare forced into the liquid, where they are rapidly dispersed bydiffusion.

[0038] Overpressure refining is based on the concept that the forcingout of the bubbles (which would already occur because of the surfacetension) is accelerated by means of the overpressure.

[0039] The methods according to the invention have a series ofadvantages:

[0040] by overpressure refining small bubbles can be immediately forcedout,

[0041] no spontaneous bubble formation (i.e. no foam) needs to beremoved,

[0042] the bubbles themselves in the comparatively high-viscosity liquid(η>about 1000 Pas) can be forced out at a sufficient pressure, thetemperature of the liquid can be clearly less than e.g. in low pressurerefining,

[0043] the materials for the refining chamber are economical(conventional furnace block),

[0044] the overpressure refining is self-stabilizing in the sense thatthe process conditions for the overpressure refining are usuallyimproved with increasing depth in the liquid because the increasinghydrostatic pressure acts beneficially.

[0045] In principle, all bubbles can be forced out by the overpressurerefining, i.e., the liquid, and also the glass melt, can be made(temporarily) bubble-free. In order to maintain the requiredoverpressure and the associated features within certain limits, theprocess is guided or controlled so that the internal pressure in thebubbles immediately under the surface of the liquid level is at leastabout 0.01 MPa greater than the sum of the equilibrium pressures of thegases dissolved in the liquid and the sum of the vapor pressures of thecomponents evaporating from the liquid.

[0046] Because of that, all bubbles are forced out whose radius is lessthan 1 mm which is sufficient for most applications.

[0047] In order to attain a sufficient refining the process according tothe invention is controlled so that the duration of the overpressureapplication amounts to at least 60 s.

[0048] The process of overpressure refining also functions when theliquid already has a viscosity of more than 10 Pas in the refiningchamber. The process is controlled so that the viscosity of the liquiddoes not drop below 10 Pas, since otherwise bubbles can again bereleased.

[0049] It is understandable that overpressure refining removes no gasfrom the liquid, even under conditions in which the equilibrium pressureof the dissolved gases increases. One must thus prevent the dissolvedgas from later forming new bubbles.

[0050] According to a further embodiment of the process according to theinvention this occurs because the liquid is cooled during the action ofthe overpressure so that its viscosity grows so that a spontaneousbubble generation is prevented after ending of the compression process.

[0051] This has the following advantages:

[0052] the viscosity of the liquid decreases as the temperature riseswhich at the same time reduces the formation of new bubbles, and

[0053] the solubility of the gases in the liquid increases and thus theequilibrium pressure drops because of the temperature decrease.

[0054] In order to remove the large bubbles from the liquid, anadditional refining step is connected with the overpressure refining ina further embodiment of the invention.

[0055] This step can be a chemical refining step. However preferablyaccording to another embodiment of the invention a physical refiningstep comprising low pressure refining is provided because of theabove-mentioned advantages of physical refining in comparison tochemical refining.

[0056] This two step refining, i.e. the combination of a low pressurerefining and an overpressure refining, in which the liquid is acted onwith a lower pressure and then with an overpressure has the advantagethat the liquid is more highly refined or is provided with a higherrefining degree without chemical influences:

[0057] in the first step the larger bubbles are removed from the melt;generally no bubbles survive and/or new small bubbles rise arespontaneously generated, and

[0058] in the second step the small bubbles are forced out by theoverpressure refining.

[0059] The condition for optimum two step refining is that theequilibrium pressure of the dissolved gases (Pgg) is less than thenormal pressure (Pn).

[0060] The change of the pressure exerted on the liquid can occur indifferent ways.

[0061] According to a first embodiment the method is performed so thatthe change of the pressure on the liquid is caused by a change in thehydrostatic pressure on the liquid.

[0062] This step can be performed in a comparatively simple manner bymeans of appropriate geometric design features of the refining chamber.

[0063] According to another embodiment of the invention the process isperformed so that pressure changes are caused in the liquid bygenerating sound waves in the liquid.

[0064] The refining of the liquid in a sonic field is a very efficientway to bring about overpressures and low pressure conditions in theliquid to physically influence the bubble spectrum.

[0065] The above object of the invention is similarly attained in anapparatus according to the invention comprising a refining chamberarranged between two processing vessels. The refining chamber is loweredrelative to the normal liquid level in the processing vessels so that ahigher hydrostatic pressure exists in it than in the processing vessels.It is at least arranged at a height or level (A) below the one upstreamprocessing vessel so that the internal pressure in the bubblesimmediately under the surface of the liquid level in the refiningchamber is at least as great as the sum of the equilibrium pressures ofthe gases dissolved in the liquid and the sum of the vapor pressures ofthe components evaporating from the liquid.

[0066] An apparatus of this type provides an effective overpressurerefinement of the liquid with a comparatively simple structure.

[0067] According to an additional embodiment of the invention theapparatus is advantageously formed so that the lowered refining chamberfor overpressure refining is connected in series to a tower-likeelevated refining chamber for low pressure refining.

[0068] This combination allows an especially effective refining. Whenthe liquid is acted on in the first chamber with a lower pressure, thelarge bubbles at the surface climb or rise and are destroyed or skimmedoff there. Subsequently in the second chamber the liquid is acted onwith an overpressure, so that a pressure in the bubbles in the liquid isproduced which is above the equilibrium pressure of the gas dissolved inthe liquid. Because the gases in the small bubbles are forced into theliquid, they are rapidly dispersed by diffusion. At a sufficientpressure on the liquid the bubbles themselves can still be refined at ahigh viscosity of the liquid (η>1000 Pas).

[0069] To perform sonic refining the refining chamber of the apparatusaccording to one embodiment of the invention has an appropriatesonotrode for generating sound waves in the liquid in the refiningchamber that allows the production of an effective sonic field.

BRIEF DESCRIPTION OF THE DRAWING

[0070] The objects, features and advantages of the invention will now beillustrated in more detail with the aid of the following description ofthe preferred embodiments, with reference to the accompanying figures inwhich:

[0071]FIG. 1 is a diagrammatic cross-sectional view of a refiningchamber for overpressure refining of a liquid, especially a glass melt,according to the invention;

[0072]FIG. 2 is a diagrammatic cross-sectional view of a refiningchamber for low pressure refining of a liquid, especially a glass melt;

[0073]FIG. 3 is a diagrammatic cross-sectional view of an apparatus forperforming a method of liquid refining according to a preferredembodiment of the invention, including a combination of the low pressurerefining chamber according to FIG. 2 with a downstream overpressurerefining chamber according to FIG. 1, and

[0074]FIG. 4 is a diagrammatic cross-sectional view through an apparatusfor performing a sonic refining of a liquid including a refiningchamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0075]FIG. 1 is a schematic longitudinal cross-sectional view through arefining chamber for physical refining of a liquid, in the presentembodiment a glass melt 2, which has dissolved gases therein andbubble-like gas inclusions, i.e. gas bubbles. The melt 2 flows from anupstream processing vessel P, a melt furnace, through the inlet 3 of therefining chamber 1 into the refining chamber 1 and then after theperformance of the refining through the outlet 4 into an subsequentlyconnected processing vessel P′. The height or level of the melt in theprocessing vessels as stated above determines the height of the inlet 3and the outlet 4. The refining chamber 1 further has a vertically upwardand downward movable control sword element 5 for control of the flow ofthe melt in the melt vessel.

[0076] For producing the overpressure, as illustrated, the refiningchamber or vessel is arranged at a lower level relative to the normallevel in the processing vessel so that a higher hydrostatic pressureexists in it than in the processing containers. The amount or depth A ofthe lowering is selected so that the internal pressure in the bubbles isat least as great as the sum of the equilibrium pressures of the gasesdissolved in the melt and the sum of the vapor pressures of thecomponents evaporating from the melt.

[0077] As already mentioned, no bubbles are forced out of the liquid bythis overpressure refining.

[0078]FIG. 2 shows a tower-like or column-like low pressure purifyingchamber 6 with an inlet 7 for the melt and an outlet 8, as well as acontrolling sword element 9 for regulating the flow of the melt. Apassage 10 is provided in an upper section of the purifying chamber 6,to which a vacuum pump is connectable in order to produce a reduced orlow pressure in the space 6 b above the liquid level of the melt 2 andbelow the top-side boundary 6 a of the refining chamber. A bubbleseparator is also connected to this passage. A vertically movablemechanical bubble barrier is arranged in the space 6 b above the melt,which particularly should prevent foam exposed on the liquid surfacefrom reaching the next stage.

[0079] In the low pressure purification in the tower-like structure thehydrostatic pressure of the melt is similarly employed.

[0080] The outer pressure on the bubble at the depth h under the liquidsurface is given by

Pa=Pao−ρg·h

[0081] wherein g=9.81 m/s². For a melt with a density, e.g. ρ=2500kg/m³, a vacuum head of 4 m is sufficient in order to completelycompensate for the surrounding pressure of Pao=10⁵ Pa.

[0082] The low pressure refinement has the above-described effect, i.e.it predominantly causes a refinement of the large bubbles.

[0083] In FIG. 3 an apparatus is shown which combines the advantageousaction of the overpressure refinement with that of the low pressurerefinement, without which the disadvantages of the latter must be borne.

[0084] This apparatus comprises a low pressure refining chamber 6according to FIG. 2 combined with the downstream overpressure refiningchamber 1 of FIG. 1. The melt is first acted on with a low pressure andthen with a high pressure in the apparatus shown in FIG. 3.

[0085] Because the melt 2 is first acted on with a low pressure largebubbles can rise to the surface and burst there or be skimmed offthrough the passage 10. Because the melt 2 is subsequently acted on withan overpressure, a pressure arises in the bubbles in the melt that isover the equilibrium pressure of the gases dissolved in the melt.Because of that the gases located in the small bubbles are forced intothe liquid where they are dispersed rapidly by diffusion. With asufficient pressure on the melt the bubbles themselves are refined atthe high viscosity of the melt.

[0086] As has already been mentioned, fresh bubble formation in therefining chamber I can be prevented by lowering the temperature of themelt.

[0087] In FIG. 4 an apparatus is shown diagrammatically which isprovided for sonic refinement of the melt 2. It comprises a vessel 12with an inlet 13 and an outlet 14, in which sonotrodes 15 and 16, alsocalled sonic electrodes, are inserted, which generate sound waves in themelt 2.

[0088] The optimum sonic frequency is in the 1 to 10 kHz range and thesound intensities are <10 kW/m2.

[0089] Periodic variations in pressure are produced by the sonic field,as also can occur by rotary stirring. The sonic refining can berepresented as a periodic or repeated overpressure refining/low pressurerefining.

[0090] The pressure amplitude of the sound waves can increase thesurrounding pressure several orders of magnitude, even at comparativelylow sound intensities. By adjustment of the parameters of the sonicfield (namely the frequency, chirp, intensity, effective duration)correctly the size of the bubbles and thus their refinement can beefficiently influenced.

[0091] For an efficient refinement it is necessary to couple the bubblesin the melt efficiently to the sonic field. It has been shown thatmainly the bubbles whose resonance frequency corresponds closely to theresonance frequency of the sonic oscillation react with the sonic field.With a suitable chip (time varying frequency) of the sonic field a broadspectrum of bubbles can be refined.

[0092] With a sonic or sound wave generator that operates at a fixedfrequency the bubble resonance frequency can be tuned to the soundfrequency by varying the hydrostatic pressure on the bubbles. This canoccur when the melt is guided through the refining chamber at differentlevels, e.g. by a division of the refining chamber according to FIG. 4by the sword barrier elements 17, 18 and 19.

[0093] The disclosure in German Patent Application 198 22 437.043 of May19, 1998 is incorporated here by reference. This German PatentApplication describes the invention described hereinabove and claimed inthe claims appended hereinbelow and provides the basis for a claim ofpriority for the instant invention under 35 U.S.C. 119.

[0094] While the invention has been illustrated and described asembodied in a process for physically refining a liquid, especially amelt, and apparatus for performing said process, it is not intended tobe limited to the details shown, since various modifications and changesmay be made without departing in any way from the spirit of the presentinvention.

[0095] Without further analysis, the foregoing will so fully reveal thegist of the present invention that others can, by applying currentknowledge, readily adapt it for various applications without omittingfeatures that, from the standpoint of prior art, fairly constituteessential characteristics of the generic or specific aspects of thisinvention.

[0096] What is claimed is new and is set forth in the following appendedclaims.

I claim:
 1. A method for physical refining a liquid containing dissolvedgases and gas bubbles and including components evaporating therefrom,said method comprising applying an overpressure to said liquid having amagnitude selected so that an internal pressure in said bubblesimmediately under a surface of the liquid in a refining chamber is atleast as great as a sum of equilibrium pressures of the gases dissolvedin the liquid and a sum of the vapor pressure of the componentsevaporating from the liquid.
 2. The method as defined in claim 1,wherein the internal pressure in said bubbles immediately under thesurface of the liquid is at least about 0.01 Mpa greater than said sumof equilibrium pressures of the gases dissolved in the liquid and saidsum of the vapor pressure of the components evaporating from the liquid.3. The method as defined in claim 1, wherein said applying of saidoverpressure occurs for a duration of at least 60 s.
 4. The method asdefined in claim 1, wherein said liquid is provided on introduction tosaid refining chamber with a viscosity of more than 10 Pas and saidviscosity is controlled so that said viscosity is not below 10 Pasduring the applying.
 5. The method as defined in claim 4, furthercomprising cooling during the applying of the overpressure so that saidviscosity of said liquid increases so that a spontaneous generation ofadditional bubbles is prevented after said applying has ended.
 6. Themethod as defined in claim 1, wherein said overpressure is selected sothat only those of said bubbles having a radius of less than 1 mm areforced out.
 7. The method as defined in claim 1, further comprising anadditional refining step in which comparatively larger ones of saidbubbles are removed from said liquid.
 8. The method as defined in claim7, wherein said additional refining step is a chemical refining step. 9.The method as defined in claim 7, wherein said additional refining stepis a low pressure refining step.
 10. The method as defined in claim 7,wherein said overpressure is provided by a change in a hydrostaticpressure in said liquid.
 11. The method as defined in claim 8, furthercomprising generating sound waves in said liquid to cause pressurefluctuations in said liquid to produce said overpressure.
 12. The methodas defined in claim 11, wherein said sound waves in said liquid have aresonance frequency tuned to a bubble resonance frequency.
 13. Themethod as defined in claim 12, further comprising exposing said bubblesin said liquid to different values of said hydrostatic pressure in orderto tune said bubble resonance frequency to said sound wave resonancefrequency.
 14. The method as defined in claim 9, further comprisinggenerating sound waves in said liquid to cause pressure fluctuations insaid liquid to produce said overpressure.
 15. The method as defined inclaim 14, wherein said sound waves in said liquid have a resonancefrequency tuned to a bubble resonance frequency.
 16. The method asdefined in claim 15, further comprising exposing said bubbles in saidliquid to different values of said hydrostatic pressure in order to tunesaid bubble resonance frequency to said sound wave resonance frequency.17. The method as defined in claim 1, wherein said liquid is a glassmelt.
 18. An apparatus for physically refining a liquid containingdissolved gases and gas bubbles, said apparatus comprising a pluralityof processing vessels (P,P′) for treatment of the liquid; and a refiningchamber (1) for overpressure refining of the liquid, said refiningchamber (1) being connected between two of said processing vessels andarranged at a level below a normal liquid level in one of saidprocessing vessels at a depth (A) such that a hydrostatic pressure insaid refining chamber (1) for overpressure refining is sufficientlyhigher than that in said one of said processing vessels so that aninternal pressure in said gas bubbles in said liquid immediately under asurface of said liquid in said refining chamber (1) for overpressurerefining is at least as great as a sum of equilibrium pressures of thegases dissolved in the liquid and a sum of the vapor pressure ofcomponents evaporating from the liquid.
 19. The apparatus as defined inclaim 18, further comprising a tower-shaped elevated refining chamber(6) for a low pressure refining connected upstream of said refiningchamber (1) for overpressure refining.
 20. The apparatus as defined inclaim 18, wherein said liquid is a glass melt.
 21. An apparatus forphysically refining a liquid containing dissolved gases and gas bubbles,said apparatus comprising at least one processing vessel (P,P′) fortreatment of said liquid; at least one refining chamber (12) forphysically refining said liquid; and means for varying a pressureexerted on said liquid relative to a pressure existing in said at leastone processing vessel, said means for varying said pressure includingsonotrodes (15,16) for producing sound waves in said liquid in said atleast one refining chamber, said sonotrodes being arranged in said atleast one refining chamber.
 22. The apparatus as defined in claim 21,further comprising means for guiding a flow of said liquid throughregions with different hydrostatic pressure in said at least onerefining chamber, said means for guiding comprising sword barrierelements (17,18,19) provided in said at least one refining chamber (12).23. The apparatus as defined in claim 21, wherein said liquid is a glassmelt.